The invention relates to the preparation of yellow bis(3-[triethoxysilyl]propyl)tetrasulfane.
From DE 21 41 159 there is known the preparation of bis(3-[triethoxysilyl]propyl)tetrasulfane (Si 69) by reaction of chloropropyltriethoxysilane with sodium polysulfide (NPS). The chloropropyltriethoxysilane that is used can be prepared by ethanolysis of chloropropyltrichlorosilane. Ethanolysis according to known processes, as described in DE 20 61 189 and DE 32 36 628, yields a completely converted product having only a very low content of chloropropylmonochlorodiethoxysilane. Such completely converted chloropropyltriethoxysilane is referred to as xe2x80x9cneutralxe2x80x9d hereinbelow.
If neutral chloropropyltriethoxysilane is reacted according to the process described in DE 21 41 159 with sodium polysulfide or with Na2S and sulfur to prepare bis(3-[triethoxysilyl]propyl)tetrasulfane, then a product that is dark yellow to red in colour (iodine colour valuexe2x89xa720 mg of iodine/100 ml) is obtained. There has been introduced onto the market, however, a light-yellow product that is not obtainable from neutral chloropropyltriethoxysilane. In order to obtain light-yellow bis(3-[triethoxysilyl]propyl)tetrasulfane (iodine colour valuexe2x89xa610 mg of iodine/100 ml), a so-called residual acid content in the form of chloropropylmonochlorodiethoxysilane must be present in the chloropropyltriethoxysilane. That can be achieved by not bringing the ethanolysis reaction completely to an end. In operational practice, however, such a measure requires a not inconsiderable additional expense, in particular because the residual acid content must be kept within a very narrow range, that is to say the reaction must be terminated very specifically at a particular point shortly before conversion is complete.
Acidification of neutral chloropropyltriethoxysilane with alcoholic hydrochloric acid before the reaction with the mentioned sulfurising agents also yields bis(3-[triethoxysilyl]propyl)tetrasulfane that is not light-yellow but likewise dark yellow to red in colour.
The disadvantage of such known processes is that they yield a product that is dark yellow to red in colour.
The object of the invention is to provide an alternative process by means of which a yellow bis(3-[triethoxysilyl]propyl)tetrasulfane is obtained.
The invention provides a process for the preparation of yellow bis(3-[triethoxysilyl]propyl)tetrasulfane having an iodine colour value of xe2x89xa610 mg of iodine/100 ml, preferably from 5 to 7 mg of iodine/100 ml, which process is characterised in that chloropropyltrichlorosilane is added to neutral chloropropyltriethoxysilane and reaction with sodium polysulfide or with Na2S and sulfur in ethanol is then carried out.
The addition of small amounts of chloropropyltrichlorosilane to neutral chloropropyltriethoxysilane can lead to the formation of small amounts of chloropropylmonochlorodiethoxysilane and chloropropyldichloromonoethoxysilane (1). 
The formation of chloropropylmonochlorodiethoxysilane and chloropropyldichloromonoethoxysilane, even in the absence of a solvent such as, for example, ethanol, can be confirmed by GC-MS investigations.
That process has the advantage that the ethanolysis reaction does not have to be terminated very specifically at a particular point shortly before conversion is complete, but can be carried out to complete conversion to neutral chloropropyltriethoxysilane. The small amount of chloropropylmonochlorodiethoxysilane that is required for the preparation of light-yellow bis(3-[triethoxysilyl]propyl)tetrasulfane can be obtained in a simple manner by adding chloropropyltrichlorosilane to neutral chloropropyltriethoxysilane.
The metering sequence is not critical. It is possible to add chloropropyltrichlorosilane to a solution of neutral chloropropyltriethoxysilane and ethanol, or chloropropyltrichlorosilane may be added to neutral chloropropyltriethoxysilane, followed by ethanol.
Chloropropyltrichlorosilane may be added in amounts of from 0.1 to 20 wt. %, preferably from 0.5 to 5 wt. %, particularly preferably from 0.8 to 1.2 wt. %, based on chloropropyltriethoxysilane.
The reaction mixture may be heated before the addition of sodium polysulfide or Na2S and sulfur, preferably to temperatures of from 20 to 90xc2x0 C.
Bis(3-[triethoxysilyl]propyl)tetrasulfane prepared by the process according to the invention has an iodine colour value of xe2x89xa610 mg of iodine/100 ml.